Mid- and far-infrared rays in a band of wavelengths of 5 to 10 μm shows strong absorption peaks corresponding to those of various gases and living cells, so that such rays draw attention in applications of measurement and analysis based on infrared ray spectroscopy. It is, however, necessary to scan in a range of wavelengths near absorption peaks of gases or the like for performing infrared ray spectroscopy. It is thus required a light source for 5 GHz or lower whose line width of spectrum is narrow and wavelength is tunable.
Although semiconductor lasers have been widely applied for gas analysis in near-infrared spectroscopy, they can be applied in a range of wavelength of 3 μm at most and cannot be applied in mid- and far-infrared ray spectroscopy.
Although quantum cascade laser can oscillate light in a range of mid-infrared rays to terahertz lights and expected in the related applications, it is difficult to realize a wavelength tunable laser and to utilize the laser in applications of spectroscopy.
On the other hand, according to systems of wavelength conversion, it is used a device utilizing quasi-phase matching (QPM) made of a material such as lithium niobate, so that it is possible to convert the wavelength from near-infrared range to mid-infrared range utilizing difference frequency or parametric generation. It is, however, difficult to generate converted radiation having a wavelength of 4.5 μm or higher due to absorption of infrared rays in the material.
On the other hand, semiconductors such as GaAs and ZnSe transmit radiations having a wavelength of 1 to 10 μm. Further, as described in Non-Patent Reference 1, it is possible to produce a QPM device (OP-GaAs: Orientation Patterned GaAs) whose crystal orientation is inverted during its crystal growth.
Then, the phase matching characteristics of the above described OP-GaAs is shown in FIG. 1. An inversion period Λ is two fold of a coherence length. It is proved to be possible to convert pump lights of wavelengths of 1 to 4 μm to infrared rays of wavelengths of 2 to 10 μm, by appropriately adjusting the inversion period Λ. Here, in formula shown in FIG. 1, “np” represents a refractive index of the OP-GaAs with respect to the pump light, “λp” represents a wavelength of the pump light, “ns” represents a refractive index of the OP-GaAs with respect to a signal light, “λs” represents a wavelength of the signal light, “ni” represents a refractive index of the OP-GaAs with respect to an idler light, and “λi” represents a wavelength of the idler light.
They are known two kinds of systems generating idler light in such devices. First, pump and signal lights are made incident into a bulk-type OP-GaAs device to irradiate an idler light. However, the conversion rate is low so that it is required to increase the intensity of the incident light. It is therefore necessary to make both of the pump and signal lights are made incident as CW lights to amplify them by means of optical fiber amplifiers.
Second, it is known to make a pump light incident into a bulk-type OP-GaAs device to generate idler and signal lights based on parametric generation (Non-Patent Reference 2). However, the conversion rate is low so that it is required to increase the intensity of the incident light. A pulse laser is thereby used as the pump light to increase the peak intensity and to improve the conversion efficiency.
It is described, in the Patent Reference 2, a typical waveguide structure of an OP-GaAs device. In the case of such type of semiconductor device, its optical waveguide is of a ridge type. For example as shown in FIG. 2, light is laterally confined by the ridge shape. Further, in the direction of the depth of the ridge waveguide, the mixing ratio of Al is changed to form a core and clad layers are formed over and under the core, respectively to realize the confinement of the light in the direction of the depth. For example, the core of Al 67 percent is formed in the direction of depth.
On the other hand, as a GaAs device having QPM structure, it is known a bulk-type device, for example, having a thickness of 500 μm
(Patent Reference 1).
Further, an infrared ray generating system by Stanford University was disclosed in Non-Patent Reference 3.
(Patent Reference 1)
    U.S. Pat. No. 6,273,949.(Non-Patent Reference 1)    L. A. Eyres, et. al., “All-epitaxial fabrication of thick, orientation-patterned GaAs films for nonlinear optical frequency conversion” Appl. Phys. Lett., Vol. 79, No. 7, Aug. 13, 2001(Non-Patent Reference 2)    Xiaojun Yu, “MBE GROWTH OF III-V MATERIALS WITH ORIENTATION-PATTERNED STRUCTURES FOR NONLINEAR OPTICS”,    A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY, IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY, March 2006)(Non-Patent Reference 3)    Thierry Jacques Pinguet, “ORIENTATION-PATTERNED GALLIUM ARSENIDE FOR QUASI-PHASEMATCHED INFRARED NONLINEAR OPTICS”,    A DISSERTATION SUBMITTED TO THE DEPARTMENT OF APPLIED PHYSICS AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY, March 2002